Adds a function to be called whenever there are no higher priority events pending.
If the function returns false it is automatically removed from the list of event sources and will not be called again.
See memory management of sources for details on how to handle the return value and memory management of
This internally creates a main loop source using IdleSource and attaches it to the global MainContext using attach, so the callback will be invoked in whichever thread is running that main context. You can do these steps manually if you need greater control or to use a custom main context.
function to call
data to pass to
function to call when the idle is removed, or null
the ID (greater than 0) of the event source.